`
https://leetcode.cn/problems/invert-binary-tree/
`

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function (root) {
  if (root === null) {
    return root
  }

  // 交换左右子树
  [root.left, root.right] = [root.right, root.left]

  // 递归翻转左右子树
  invertTree(root.left)
  invertTree(root.right)

  return root
};